In the examination of microscopic specimens in which dynamic processes that can be stimulated externally occur, individual regions of the specimen are often of particular interest. The stimulation can refer to initiation, termination, or any other influencing of a process.
In cell biology, the transmission of information from cell to cell is studied. The nerve cells branch among and are in contact with one another. At the contact points, so-called spines that are located on the dendrites of the cells are linked to the synapses of another cell. A method for locating the contact points is based on preparing the nerve cells with a stain, for example a calcium indicator such as “calcium green,” that emits a characteristic fluorescent light when illuminated with light of a suitable exciting wavelength. By stimulation with a voltage pulse that is applied to the cell tissue, it is possible to induce the transition to an action potential that propagates from cell to cell. This is associated with an inflow of calcium into the spines that is detectable by way of the calcium indicator. One such method is known from the article “Ca2+Fluorescence Imaging with Pico- and Femtosecond Two-Photon Excitation: Signal and Photodamage,” Helmut J. Koester et al., Biophysical Journal, Vol. 77, October 1999, 2226-2236.
The known method has the disadvantage that the contact points cannot be reliably and reproducibly localized; the reason is that, because the processes at the contact points take so little time, only a few indicator molecules are excited and therefore only a few characteristic photons are generated, yielding a very poor signal-to-noise ratio. Locating the contact points is therefore a very laborious process, and requires a great deal of experience and intuition on the part of the user.